Many industrial process instruments operate on a two wire control loop with a current that varies from 4-20 mA based on a sensor reading or a desired actuator condition. In the case of a sensor, a host connected to the process instrument determines the measured value by measuring the control loop current. In the case of an actuator, the control room supplies a current to the process instrument which indicates a desired actuator condition.
The host is located in a control room and supplies approximately 24V DC to the two wire device. For a sensor, simple diagnostics can be accomplished by measuring an out of range current such as 3.5 mA or 20.5 mA. There may be as much as a mile or more of cable between the control room and the device causing a small voltage drop from the resistance of the wires. Electronics in the device regulate the voltage to a nominal value such as 12V to power a sensor and a microprocessor.
The microprocessor makes the sensor measurement and determines the necessary current value. It uses a digital to analog converter (DAC) to control a control amplifier and control transistor to consume current through a shunt resistor such that the total current draw of the electronics and the shunt resistor is the proper value. A feedback loop is completed using a high precision sense resistor that measures the total current usage of the process instrument to be sure an accurate value is reported.
Traditional design techniques for process instruments specified that only functions which could be accomplished with the minimum current (3.5 mA) would be implemented. This is due to the nature of the environment in which a process instrument operates. These devices are very low power, often installed in remote locations, and could bring entire operations to a halt if they fail. Therefore, it is essential that the process instrument be fully functional at the lower limit of the available power.
Although the device must operate at 3.5 mA, it may be operating as high as 20 mA. This means that 16.5 mA or more of available power is being thrown away in the shunt resistor.
One use for this current is to provide LED backlighting for the process control instrument. A past approach to provide this feature was to replace the shunt resistor with an LED. While this does provide for backlighting, there is no control of the intensity of the backlight. At 4 mA, the backlighting is dim, while at 20 mA, it can be overly bright.